


Thrown for a Loop

by Askellie



Category: Undertale (Video Game)
Genre: Fluffy, Incest, Light Sadism, M/M, Multiple Orgasms, Overstimulation, Pap is a sadist, Sacrum Lacing, Sensation Play, Teasing, but the nicest one you'll ever meet, consensual wrecking of Sans's ass
Language: English
Status: Completed
Published: 2019-12-04
Updated: 2019-12-04
Packaged: 2021-02-26 03:01:25
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 4,273
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/21666352
Author URL: https://archiveofourown.org/users/Askellie/pseuds/Askellie
Summary: Sans thinks Papyrus just wants to try a little, harmless kink. No big deal; he'd agree to anything to make his bro happy.Turns out he's underestimated both Papyrus's intentions, and his ingenuity with what can be achieved with just a few pieces of string.
Relationships: Fontcest - Relationship, Papyrus/Sans (Undertale)
Comments: 10
Kudos: 255





	Thrown for a Loop

**Author's Note:**

> A commissioned fic for the amazing @symphoneum on Twitter, who gave me the delicious prompt of sacrum lacing and Sadist!Papyrus. :9 This piece speaks pretty strongly to my heart, because as a sadist myself, there's no greater delight in convincing your partner to suffer sweetly for you.

When Papyrus brought out the ribbons and shyly asked if Sans was up for trying something new, Sans had agreed without a second thought. The ribbons were soft silk, thin and delicate, coloured red to orange like the surface’s sunset. Sans figured if his bro wanted to get his kink on and try out a little bondage with ties that weren’t as daunting as rope or handcuffs, it was a good first step. A nice safe and gentle introduction to see if it was something they might want to explore more seriously in future. His brother was always so considerate, especially when it came to Sans’s physical fragility.

“Where do you want me?” Sans asked, surveying Papyrus’s room. All the action figures had been cleared away, temporarily exiled to the closet (“There’s some things they don’t need to see, Sans!”) and the bed had been made up with the special sheets that were just the right shade of blue to hide the stains Sans tended to leave behind (Sans liked to think of them as the sexing sheets, though Papyrus squawked in mortified outrage every time he called them that).

“On your hands and knees,” Papyrus said, staring at him with that bright-eyed, eager look, like Sans was a Giftmas morning present he was about to open. “Bare-bones, please.”

Sans grinned and shucked off his clothing, throwing them into the corner where Papyrus could hopefully ignore the temporary mess. He realised a moment too late that he should have made more of a show of it, a little strip tease to wind his brother up, but from the looks of things Pap didn’t need any more encouragement. There was a shimmer of orange peeking from the shorts of his battle body, casting sunny highlights on his iliac crests. Sans had to swallow down the sudden rush of saliva in his mouth as he complied with Papyrus’s request, easily offering his upturned tailbone as he faced the wall.

“Like this?”

“Perfect, brother.” Papyrus’s voice was a veritable purr and the praise made Sans’s soul do a giddy flip in his chest cavity. He fought the urge to turn and watch as Papyrus came up behind him, the warmth of his shadow falling over Sans’s bones like an intangible caress. “Now hold still for me.”

His hands settled on Sans’s hips, causing the smaller skeleton to twitch like he’d been zapped with a live wire. He gave a bashful laugh, trying to convince his tightly-strung bones to relax. He wasn’t afraid of Papyrus, by any means. It was more of a nervous excitement. He’d been ready to play it cool while Papyrus explored this new side of himself, but the unflappable patience he normally wore so easily was nowhere to be found. “Sorry, Pap. Guess I’m a little strung out.”

Papyrus gave a disgruntled sigh, his fingers tightening on Sans’s illum with a pinch of gratifying pressure. “What have I told you about the appropriateness of bedroom puns, Sans? I’m going to remember that for later.”

“Welp. Hopefully I’ll still be able to string together an apology by then.”

This time, the pun earned him a gentle smack across the back of his sacrum, eliciting a yelp. He was grinning, though, face flushed a bright blue that no doubt matched the swirl of magic starting to work its way down into his pelvic cavity. Papyrus had barely touched him yet, but anticipation had him primed. 

He gave his hips an enticing wiggle. “You want me to give you anything to work with down there?”

“Not yet,” Papyrus told him, stern and thoughtful. “In fact, I don’t think I shall give you permission to form anything until I feel you’re appropriately sorry for your misbehavior.”

Sans would readily admit that he was more than happy to follow Papyrus’s lead in the bedroom. Sex was a puzzle that Papyrus was intently and enthusiastically figuring out for himself, and Sans was content to lie back and let his brother do most of the work. There was no reason for the simple order to cause Sans’s magic simmer in his bones, making him feel overheated even without his clothes. It took two tries before he could be sure his voice wouldn’t shake when he spoke. “Sure, but, y’know, you might be waiting a while. I’m not feeling very sorry.”

“We’ll see.” Papyrus’s tone was entirely benevolent, almost enough to hide the dangerous smugness beneath it as he felt his way down the back of Sans’s pelvis. His gloved fingers traced the bumps and notches of Sans’s sacrum, exploring the holes with more interest and attention than Sans had ever given them himself. His breath caught in his throat, his body struggling against the impulse to arch away from how sensitive they felt at even that light exploration.

“Now, prepare yourself, brother,” Papyrus ordered, sounding nothing at all like the squirming, embarrassing mess that had coyly asked Sans about the ribbons. “I believe this will feel somewhat unusual.”

“What do you mea-aaaaaahhhhh, oh fuck,  _ oh fuck _ -!”

Papyrus deftly thread the tip of a ribbon through the top hole of his sacrum, and eased it through from below in a long, steady pull that made Sans’s thoughts collapse like a house of cards, descending into a fluttering, chaotic mess. The slide of the silk almost burned, even with how smooth the ribbon was, but it was the good kind of burn, like the aching stretch of his cunt the first time he’d managed to take the entirety of Papyrus’s dick. It was a feeling of overwhelming fullness and near-painful friction right on his most delicate nerves, making him shake from his skull to his toes. He was barely aware of Papyrus crooning approval at him, his normally booming voice softened with care. Nothing penetrated around the mind-blowing feeling of the ribbon moving through him in an intimate, endless glide.

Though not truly endless. It only felt like it had been years of senseless assault before Papyrus finally stopped tugging, leaving the ribbon to dangle freely. The stillness was almost worse than movement had been, and Sans let out a plaintive whine, hips bucking in a futile effort to make it shift again. If Papyrus’s hand wasn’t still lodged beneath his tailbone, Sans’s magic would have already dropped into place, desperate and swollen and invitingly wet. He ached with how much he wanted it, and it took him a few disoriented seconds to remember why he wasn’t allowed. 

“That was a very nice sound you made,” Papyrus observed, sounding pleased. Sans could only blink dazedly. He hadn’t even heard himself making noise, but now he could feel the resonance in the back of his throat, a hoarse, wet echo still fading out. “Are you ready for the next one?”

Sans made an unseemly gurgle of disbelief, but wasn’t enough of an objection to make Papyrus hesitate. He took the dangling ribbon and pushed it though the next hole, plucking the ribbon from the top to draw it into a tight, horizontal line on the inside of Sans’s sacrum. This time, he pulled with such slow deliberation it was almost torture, excrutiating in the most maddening way. This time, Sans was mortifyingly aware of the obscene timbre of his moan as it spilled helplessly from between his teeth. His arms shook, and after a moment he had to let them fold, face planting down into the sheets. His legs felt equally strengthless, but they were locked in a shock of paralysis. He could barely feel them at all; nothing existed outside of that long, burning, brilliant tug that made his toes curl and his eye-lights roll back into his skull. 

When it had been pulled though to Papyrus’s satisfaction, Sans was granted a small reprieve, struggling to catch his breath and wonder where the hell his brother ever got the idea to try this. 

“I feel this is a good time to remind you that we did come up with a safe word for just this occasion,” Papyrus told him kindly. “And if you feel the need to use it I won’t be at all offended or disappointed!”

Sans had thought the safeword was a silly, unnecessary precaution. If anything were to truly go wrong, Papyrus would feel it in the ripples of Sans’s magic much quicker than Sans could blurt out a word, but the dating manual had insisted that it was a good practice in case of distraction or misunderstanding. Sans’s head was spinning, and for a wild moment he wondered if he should actually tap out in case his body wasn’t capable of living up to the finale of this little experiment. His bones were still rattling with aftershocks, and his mind felt like the aftermath of an earthquake, everything shaken loose and left in chaotic disarray.

But he didn’t want to stop.

“No?” Papyrus asked after a generous pause, notably devoid of any protests from Sans. “Okay, then! We shall continue!”

He could feel a tug as Papyrus pulled on the ribbons, drawing them tight and winding around each other in an insidious tangle that ensured they couldn’t loosen again. “I’m going to start with the next hole, brother. Try not to wriggle around too much.”

Sans had a feeling the words were more of a tease than an admonishment. Papyrus certainly didn’t seem deterred when Sans howled at the next intrusion of the ribbon, clawing mindlessly at the sheets and fighting the urge to pull away from the stimulation. Even though his magic hadn’t formed, he could feel it condensing wetly inside his pelvis, fervently lubricating everything as if that might help speed things along. The ribbon grew wet as it slid against him, and the sound of wet silk straining to move was going to be forever embedded in his mind as utterly pornographic. 

“Oh fuck, Papyrus-!” he cursed against the sheets. His sockets burned, and he wondered if his eye-lights were burning themselves out like fireworks from the sheer intensity of sensation. Maybe that was why his body suddenly turned on the waterworks, tears blurring his vision as if to keep the sparks from spitting out and setting the whole bed ablaze. “Pap, Pap,  _ please- _ !”

“We’re not even halfway done, Sans,” Papyrus scolded him cheerfully, already threading the next hole. The dual sensation of a second ribbon starting to squeeze through his bones made Sans wail incoherently. “And I do believe I said we would continue until you were feeling more apologetic. Are you?”

“Yes!” Sans gasped. “A-absolutely. No more puns in the bedroom, I swear, just please-!”

“Hmm.” Papyrus took a moment to consider his brother, the ribbons momentarily stilled. Sans couldn’t decide if that was ecstasy or agony as he feverishly waited for his brother’s judgement. “While I’m glad to hear you finally agree with me on the pun situation, your newfound repentance doesn’t excuse you from your punishment.”

Distantly, he could feel Papyrus give his femur a reassuring squeeze, his touch radiating affection and delight and _ lust lust lust _ before he gave both strings a sudden, powerful tug that brought its entire, sinuous length flush against his tailbone in a stranglehold that made Sans’s vision white-out. His magic roiled like a tidal wave, a drowning tide that rolled him under with a concussive pleasure. It was the most devestating orgasm of his life, merciless and cruel in how long it lingered, his body only coming down in slow, clumsy increments that left him feeling as displaced as a fish out of water. He was still twitching slightly, gaping uselessly, trying to breathe when the air suddenly felt thin and unsatisfying. 

Blinking dazedly, he suddenly realised he was on his back instead of his front. His pelvis felt wet and used even as his magic buzzed angrily, displeased with having been denied the opportunity to form. Sans had tried to get off with just his bones before (as an act of laziness rather than curiosity) but had found the effort more struggle than it was worth. He’d wondered if it was even possible to come without purpose-built genitals, but Papyrus had resoundingly answered that question for him. 

(His brother was so cool.)

Papyrus was crouched over him, staring down at Sans as if he were doing his best impression of the Judge, trying to see through Sans and look right into his soul. He beamed when he saw Sans’s eye-lights finally focus on him, reaching down to nuzzle his brother’s jawbone. “Wowie, Sans. You really liked that.”

“Y-yeah,” Sans croaked, unable to deny the way his body was still tingling. Words felt impossibly difficult for his heavy tongue, all wit and cleverness driven from his mind, replaced only with animal want. Papyrus’s nearness, his warmth, the smell of his bones, wasn’t helping Sans’s libido calm down any either, and he clumsily reached up to try and pull Papyrus down more firmly against him.

“Ah ah!” Papyrus chided, easily avoiding Sans’s graceless attempt. “I wasn’t finished yet.”

He gestured down at Sans’s pelvis, and Sans was able to get his first look at Papyrus’s handiwork. Two different ribbons had been threaded between his sacrum holes, creating a cross-laced pattern, as tight and neat as the tripwires Papyrus used for his puzzles. The colors of the silk had been muddied with Sans’s magic, their bright hues turned to a slimy green that only emphasised how very aroused Sans still was. The entire circumference of his pelvis was slicked with blue, and in the centre of the cavity was a swirling haze of mist already starting to thicken once again.

“You sure you don’t want to just…?” Sans made a meaningful gesture over his pelvis, giving Papyrus his best puppy-dog sockets. He’d just cum, but he still felt like Papyrus had been edging him for hours instead of mere minutes. His body ached with the need for Papyrus’s cock to fill him up and bring him to a more manageable and familiar climax. 

“It’s not proper to leave a job half-finished, Sans!” Slyly, Papyrus slid his fingers beneath Sans’s pubic bone and traced the circumference of the next set of holes. His gloves turned dark at the fingertips, soaking up Sans’s fluids, and Sans’s face burned as he rocked helplessly into that deliberately teasing touch. “I think your tailbone will look very nice when I’m finished. Don’t you want to see it?”

Yes. No. Sans couldn’t think of anything while Papyrus touched him like that, but instinctively he knew that if Papyrus was using that particular, coaxing tone, then surrender was the only option.

“I guess…” 

It may not have been the most enthusiastic agreement, but the expression on Papyrus’s face was worth it. Papyrus took up the dangling threads of the ribbon again, and gifted Sans with a mischievous look. “You can even watch this time, if you like.”

To reach his sacrum holes from this angle, Papyrus had to slide both hands into Sans’s pelvic cavity. Even though he was very careful not to touch any of the surrounding bone, Sans was faintly trembling just from sheer anticipation. His magic swirled lovingly around Papyrus’s palms and fingers, affectionate as a hungry cat and there was probably a better pussy joke to be made there, but he couldn’t think of anything suitably clever while Papyrus was so very close and yet not nearly close enough.

“Hold still,” Papyrus commanded, quite unnecessarily. Aside from the subtle quivers he had no control over, Sans was immobilised, enraptured by the sight of Papyrus straightening out the slick end of the thread and carefully guiding it through the next sacrum hole. The first orgasm had eased the burning sensitivity of it, but the quiet hiss of it squeezing through the wet hole made a loud, shameless moan tear from his throat. He managed to keep his back from arching, trying to follow his brother’s direction, but his skull fell back as his eyes rolled towards the heavens, his breath rattling out in a shaky exhale. 

“That’s right. You’re doing so well, Sans!”

“How…” Sans struggled to string words together, not daring to look down again. He didn’t think he could take the illicit sight of the ribbon sliding through him again as well as the feel of it simultaneously. “H-how much longer?”

“Just a few minutes,” Papyrus told him, already moving to thread the next ribbon. His voice had that slightly distracted quality that came whenever he was working on a puzzle, thoughtful and full of concentration. When Sans dared to peek down again, he could see the ribbons wound around his brother’s phalanges in a complicated cat’s cradle as he worked on the lacing. Then he pulled sharply on one of the strings to tighten it into place, and Sans had to hold back a wail, fixing his gaze on a nice, safe, non-arousing patch of ceiling. “Spread your legs more, Sans.”

Sans whimpered, but complied, parting his femurs until he could feel the faint stretch of the magic at his hip joints. Papyrus settled more comfortably between them, bending over his work with a serious expression that put his mouth dangerously close to Sans’s pubic symphysis. Sans could feel the warmth of his breath, a little faster and more forceful than was justified for simple academic interest in figuring out how to craft the perfect wrapping for Sans’s tailbone. Sans’s magic apparently felt the same, because a little tendril of mist wriggled its way free of the tangled vortex in his pelvis and licked pleadingly at the edge of Papyrus’s mandible. He waved it away with a shooing finger. 

“Not yet. I thought patience was your main trait, brother.”

“Not when I need you,” Sans confessed hoarsely, the dizzy intoxication of Papyrus’s ministrations making him more honest than he usually dared to be. Embarrassingly sentimental as it was, it was worth it for the way Papyrus’s face softened, his eye-lights hazing with the adoration that had forced Sans to confront the undeniable reality that his brother’s childish crush was actually something more substantial, a feeling to match the shameful devotion Sans had harboured in his own soul since his brother had grown out of stripes.

“Well,” Papyrus said, sounding faintly flustered, his fingers working at dexterously at the ribbons, looping and tying with impressive swiftness. “Then you are in luck, for I am here to answer all your Papyrus needs! And also your fashion needs! Look!”

Somehow, in the space of a few words, Papyrus had finished with the ribbons, threading them through each hole in Sans’s sacrum and then tying them off in an absurd bow just above his coccyx. He’d crafted an intricate cross-hatch with the two different colors, twining them against each other in an elegant braid, though the effect was slightly tarnished by the uneven splotches of bright blue fluid staining them.

“Fashion, huh?” Sans asked, faintly amused. It wasn’t exactly the kind of accessory he’d want to show off at Grillby’s bar.

“It’s very cute, Sans,” Papyrus told him primly. “Just like you!”

Every so often, Papyrus blindsided him with a comment that was profoundly simple and utterly world-shattering. Sans spluttered, trying to think of a suitable objection, but Papyrus was already crawling up his body, pushing Sans back flush against the sheets. 

“Now, I appreciate how patient you have been.” The words were murmured heatedly against Sans’s mouth in a tone almost as unfair as the compliment. “So I believe it’s time to reward you.”

Papyrus’s pelvis rested heavily atop his own, still frustratingly clothed, but finally exerting some gratifyingly firm pressure. Sans bucked up against him, scrabbling fervently against the smooth chestpiece of Papyrus’s armor until his brother forcibly pinned his wrists back against the mattress. Sans’s magic surged inside his pelvis, but prickled warningly when it came up against the foreign strings looping around his sacrum. 

“Hnngh, Pap...can you...untie…?”

“After all that work?” Papyrus said, sounding faintly scandalised, though his expression was more of a smirk. “They’re not really in the way, are they?”

Sans’s magic was disgruntled by the strings, but pulled flush against the bone they didn’t provide much impediment to where it wanted to form. Sweat trickled down Sans’s bones as his magic shifted and pushed against the ribbons, pulling them even tighter as his ecto-flesh swelled into place. Even that simple act of letting his pussy finally manifest was enough to push him to the brink of orgasm, the ribbons squeezing viciously around his tailbone in a way he’d never experienced and wouldn’t have expected to enjoy. 

“That’s right, brother,” Papyrus approved. It was a good thing he was the current laundry champion of vanquishing stubborn stains, because otherwise his gloves would be a lost cause after cupping the soft mound of Sans’s cunt. The wet fabric rubbed coarsely against him, making Sans shudder in urgent want. “Let me have you.”

The fervent gush of arousal oozing out of Sans spoke of how very much in favour he was of that idea. He writhed impatiently as Papyrus trapped both his slim wrists with one big hand and used the other to skim down his shorts. The length that formed was a warm orange to match one of the ribbons Papyrus had used earlier, and thick enough that Sans always felt a small tremor of uncertainty even though he already knew his body could handle it. It helped that he was more than ready, nearly desperate to have it in him. He stretched his femurs a few more increments apart, spreading himself in an obscene invitation which Papyrus readily accepted.

Papyrus’s soft groan as he sank into Sans was nothing compared to the near squeal Sans made. The constriction of the ribbons tightened ruthlessly as his magic was stretched further, his engorged magic fighting for space in his pelvic cavity against the breadth of Papyrus’s shaft and the loops and knots of the fabric. Taking the entirety of Papyrus’s cock was always challenging, but now that feeling of unrelenting fullness pushed him right to the brink without reprieve. Sans babbled senselessly, sockets overflowing with hot tears of pure, undiluted pleasure, a feeling too immense to fully grasp. The safeword briefly crossed his mind before being recklessly swallowed beneath mindless need. If Papyrus’s steady weight wasn’t holding him down, he felt like he might fly to pieces, broken apart by the overstimulating ecstasy. 

Papyrus’s thrusts were shallow, like he couldn’t bear to pull free of the desperate grip of Sans’s pussy, and each one just ratcheted up the pleasure further until Sans couldn’t tell whether he was still in climax or just riding the brutal edge of it over and over again. His body was convulsing, struggling on sheer survival reflex to lessen the crushing tide of sensation, but Papyrus held him down with ease and drove into Sans with an intensity that bordered on torture. There was no way he could be unaware of how overwhelmed Sans was, but he didn’t so much as pause, watching Sans writhe beneath him, sobbing and wailing with eye-lights that burned like stars, fathomless and beautiful. 

Papyrus’s stamina was immense, and his control over his magic was impeccable. In the very distant corner of Sans’s mind, the final shred of his sanity and sense realised that Papyrus could very well keep him like this -- for minutes, for an hour, for as long as he wanted to, until Sans screamed himself hoarse and could barely move, nerves and magic burned out from exhaustion. That (wonderful, terrible) thought finally broke him, and he garbled out some approximation of his safeword even as his body finally lost balance on the edge of that incomprehensible abyss. He felt like an atom inside a nuclear reaction, all that burning conflagration suddenly colliding in a frisson that exploded with such force his mind went utterly blank. It was like being hit with a concussion, his sense of self flying apart until he felt almost entirely removed from his body, but instead of being rooted in pain, he drifted on a cloud of complex euphoria, thoroughly drained and satisfied beyond all measure.

He lost all track of time. It was probably several minutes before he finally came back to himself, blinking out of his stupor to find himself draped limply over his brother’s lap, being cradled with adoring care. The lack of pressure in his pelvis told him that his magic had fizzled out at some point, and the ribbons had been removed. Each sacrum hole pulsed faintly, still throbbing with the memory of being filled. 

“You were so wonderful, Sans,” Papyrus was telling him, his voice choked with pride and gratitude. “You did so well. You’re so beautiful! I love you, I love you-!”

“Love you,” Sans echoed raspily, simply because he couldn’t let those words go unanswered, even though his throat felt like it had been scraped raw. His magic was exhausted, the connection between his bones feeling loose and elastic, aching faintly, but the faint discomfort barely registered against the sheer happiness Papyrus was radiating. They were still locked pelvis to pelvis, and there was enough orange splatter on Sans’s femurs to guess that his brother had found his release as well despite Sans’s use of the safeword.

...or maybe he’d found it  _ because _ of the safeword? There were questions he was going to have for his brother later about that look in his sockets when Sans had been sobbing beneath him, but that could definitely come later. Right now, he was utterly tapped out and content to bask in Papyrus’s loving embrace and the faint, invisible bruises the ribbons had left behind.


End file.
